(1) Field of the Invention
The present invention relates to a silicon nitride sintered body and a process for the preparation thereof. More particularly, the present invention relates to a silicon nitride sintered body which is excellent in strength and creep resistance at high temperatures and a process for the preparation thereof.
(2) Description of the Prior Art
A sintered body composed of silicon nitride has mainly covalent bonds as the bonding manner of atoms and is excellent in strength, hardness and thermal and chemical stability. Accordingly, this sintered body has been used in engineering ceramics especially for a heat engine such as a gas turbine engine.
It is considered that with recent increases in the efficiency of heat engines, the operation temperature will be elevated to 1400.degree. C. or higher, and development of a material capable of being used under such high temperature conditions is desired.
As the sintering methods for the production of silicon nitride, there have been adopted a hot press method and an atmospheric pressure sintering method, and a gas pressure sintering method is recently examined.
In these methods, it has been intended to leave the added composition in the vitrious phase or crystal phase of the grain boundary or in the solid state dissolved in the crystal phase without decomposition after sintering.
However, if the added composition is thus left in the sintered body, this results in changes in excellent characteristics owing to peculiar covalent bonds of silicon nitride, such as high strength and creep resistance at high temperatures. Namely, when a sintering aid composed mainly of an oxide is added, the sintering aid reacts with silicon nitride and a metal compound composed mainly of an oxide and/or nitride of silicon is formed. This oxide and/or nitride promotes densification at the sintering step and improves the sintering property. However, since the oxide and/or nitride per se has a strong ionic bonding property, the oxide and/or nitride is likely to degrade characteristics of the sintered body at high temperatures.
As means for eliminating this disadvantage of the silicon nitride sintered body, there has been developed a DuO-density rotor by joining an inner portion (central portion) of a part to be exposed to severe application conditions, such as a rotor of a gas turbine engine, which is composed of hot-pressed silicon nitride, with an outer portion composed of reaction-sintered silicon nitride.
According to this proposal, the outer portion to be exposed to an oxidative atmosphere at a high temperature close to 1400.degree. C. is formed of a material excellent in oxidation resistance and the inner portion which is maintained at a low temperature but is subject to a large centrifugal force and connected to a shaft is formed of a material having a high strength.
This method is not practical because the manufacturing cost of the sintered body is large and the productivity is low, and the method is not suitable for mass production.
Moreover, the step of joining sintered bodies is necessary and precise joining is required, and therefore, the preparation process is complicated.